Motor/generator devices are often used in conjunction with a flywheel to perform the function of power supplies for electrical apparatus. An external power source is utilized to drive the flywheel up to speed. Upon disconnection of the power, the flywheel drives the device as a generator to produce the electrical power output. One conventional motor/generator is a brush type d.c. motor which uses a permanent magnet or wound coil for field excitation and a wound rotor armature. For operation as a generator the field power is raised so that the generator voltage exceeds the load voltage.
Among the shortcomings of such devices are the limited life of the brushes and the added resistance that the brushes introduce. Other problems derive from the centrifugal force developed on the rotor which stresses and may tend to destroy the winding and commutator. Often the armature is made small to avoid destructive centrifugal forces; in that case a separate flywheel is needed.
Also used are brushless permanent magnet rotor machines, which rotate the field to develop the torque for motor operation and drop the field excitation for generator operation. But permanent magnets are typically brittle, hard to machine and have poor tensile strength so they may fly apart under sufficient centrifugal force. The adhesive that holds the magnets is likewise affected by centrifugal force. The energy that can be inertially stored in a material used as a flywheel or rotor is limited by the yield strength of the material and the yield strength of permanent magnet material is low.